Persistent pressure overload (hypertension or valvular disease) of the adult heart leads to cardiac myocyte electrical and mechanical dysfunction. These defects can ultimately cause lethal cardiac arrhythmias (sudden death) and poor cardiac pump function (heart failure). The objectives of this project are to determine the role of new myocyte formation, myocyte turnover, and myocyte senescence in the myocardial response to pathological pressure overload. Only recently has there been evidence that new myocytes are being made in the adult heart and that these new cardiac myocytes are derived from a resident cardiac stem cell. The aims of this research are 1) To identify and characterize "newly formed" myocytes (in or recently in the cell cycle) in the adult feline heart and to identify and characterize newly formed cardiac myocytes (in culture) derived from resident cardiac stem cells; and 2) To determine the contribution of newly formed myocytes to the changes in cardiac structure and function (compensated LVH versus HF) induced by slow progressive pressure overload. Cardiac stem cells and cardiac myocytes will be isolated from normal, hypertrophied and failing hearts. The role of T-type Ca channels in the differentiation of cardiac stem cells into cardiac myocytes will be studied in culture. Similarly, the existence of T-type Ca channels in newly formed adult myocytes (labeled with cell cycle markers: BrdU and Ki67) will be explored. The electrophysiological and Ca handling characteristics of newly formed myocytes will be compared in normal, hypertrophied and failing hearts. The working hypotheses of the proposal are that the heart contains a population of resident stem cells that are capable of forming new myocytes and that the rate of new myocyte formation can be increased in response to hemodynamic stress. A specific hypothesis is that Ca influx through T-type Ca channels is essential for differentiation of new myocytes from cardiac stern cells. Heart failure is the leading cause of death in the US. A central feature of this syndrome is an increased rate of myocyte death, resulting in an insufficient number of myocytes to pump blood normally. This research explores the idea that a mechanism for replacing damaged myocytes with new myocytes with normal function exists in the normal heart. Our long term goal is to exploit the knowledge gained from the studies proposed in this application to develop therapies to repair diseased human hearts with new, normally functioning myocytes.